1. Field of the Invention
The present invention relates to the recovery of metal values from cermet material, especially cermet material of which inert cermet positive and negative electrodes (anodes) are comprised. Such inert cermet anodes include inert or non-consumable electrodes used in the production of aluminum by electrolytic reduction of alumina dissolved in a molten salt bath. In particular this invention pertains to a composition comprising fired and/or unfired cermet in a form suitable for the recovery of metal values therefrom in a smelter, especially a nickel or copper smelter, and to a smelting process which uses this composition as feedstock by itself or with ore and/or ore concentrate.
2. Background Information
Aluminum has been produced using the well known Hall-Heroult cell since Charles Martin Hall's invention for a process of reducing aluminum from its fluoride salts by electrolysis which is the subject of U.S. Pat. No. 400,664 issued on Apr. 2, 1889. In this electrolytic reduction process aluminum oxide (e.g., alumina or Al2O3) is dissolved in a bath of molten salt. The aluminum content of the alumina is reduced to metallic or elemental aluminum by an electrolytic process in which the aluminum of the aluminum oxide is reduced at the anode whereby metallic or elemental aluminum is produced. For many years carbon anodes were used in this process. The carbon anodes are consumed in the process as the carbon reacts with the alumina to produce elemental aluminum and carbon dioxide during electrolysis.
Recently inert anodes have been introduced for use in electrolytic production of aluminum. These inert anodes have the advantage of not being consumed during the reduction of the aluminum. Consequently these inert anodes are also referred to as non-consumable anodes or as dimensionally-stable anodes.
The inert or non-consumable anodes must be able to withstand the harsh conditions in which they are used (i.e., a molten salt bath which contains dissolved alumina). Furthermore, since these anodes are not consumed during the process for making aluminum, they must withstand these extremely harsh conditions for a considerable length of time. In particular the inert anode material must satisfy a number of difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte which is typically used in the Hall-Heroult process. The anode material must not react with oxygen or corrode in an oxygen-containing atmosphere. This material should be thermally stable at temperatures of about 1000° C. and should have good mechanical strength. The anode material must have electrical conductivity greater than 120 ohm−1cm−1 at the smelting cell operating temperature about 950°–970° C. In addition, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent.
Inert anodes made from cermet material have been found to satisfy the above-mentioned conditions, thus making them particularly suitable in the Hall-Heroult process.
Cermets are composite materials which have a ceramic phase and a metallic phase. They have the unique property which combines the desirable features of ceramics and metals including chemical inertness and electrical conductivity. Examples of inert anodes made from a cermet are described in U.S. Pat. Nos. 5,865,980 and 6,030,518, the specifications of which are incorporated herein by reference.
Because of the extraordinarily harsh operating environment of the cell, eventually these inert anodes made from cermet need to be replaced. Replacing the used anodes with new ones has created a disposal problem with a loss of the valuable metal components thereof. Since a typical inert anode contains combinations of metals that may include nickel, silver, copper and iron, disposal of these anodes represents a significant loss to the aluminum industry if these metals are not recovered and either sold or recycled. An inert anode described in U.S. Pat. No. 5,865,980 contains 14 wt. % copper, 7% silver, 40 wt. % nickel oxide, 38 wt. % iron and traces of other metals. Thus disposing of these anodes without recovering the metal values therefrom will be wasteful and economically disadvantageous.
Oxides of tin are also found in some inert anode materials (JOM Light Metals 1996, “Inert Anodes for the Primary Aluminum Industry” by Rudolf Pawiek, and JOM Light Metals, May 2001, “Cell Operations and Metal Purity Challenges for the use of Inert Anodes” by Thoustad and Olsen”).
The composition and characteristics of inert anodes which are used in the aluminum producing industry are discussed in an article in JOM Light Metal Age, February 2001 by Joseph Benedyk. It is noted in this article that the cermet consists of a ceramic phase and a metallic phase wherein the ceramic phase may be a matrix of nickel ferrite having a dispersion therein of a metallic phase which, for example, may be a nonferrous alloy such as copper or silver.
In addition to the used cermet anodes, there are also waste cermet anodes due to breakage, cermet ingredient materials and residues produced during the manufacturing process of the inert anodes and inert anodes that have failed to meet quality control standards. The same problems noted above with respect to the used anodes, also applies to the waste cermet associated with the above-identified materials. Thus the above-noted problems apply to used and unused inert anode and the manufacturing residues.
Although it is highly desirable to recover the valuable metals from the above-noted anode materials, no one has ever suggested any economically feasible method for their recovery, despite the need in the industry for solving this problem. This is believed to result from the fact that the inert characteristic and other characteristics which make these anodes resist the harsh conditions within an electrolytic aluminum reduction cell, make the recovery of metal values from these anodes extremely difficult and challenging. Prior to this invention no economically viable methods were known for recovering the metal values from these inert anodes. It has now been discovered by the inventors that the metal values from these inert anodes and anode materials may be economically recovered by smelting, especially in a conventional nickel or copper smelter, by converting the cermet of the inert anodes into a composition which can be smelted in the smelter.
Rath in U.S. Pat. No. 4,119,454 discloses a method for recovering ferrous metal values from steel scrap. The process employs a smelting step in which the steel scrap is fed into a smelter which produces a slag layer on top and a molten layer underneath the slag layer. The process provides for the separate recovery of the slag and metal layers. Rath does not disclose or suggest the recovery of metal values from cermet material in general nor specifically from inert anodes which comprise cermet. Furthermore, Rath does not disclose or suggest a cermet composition in a form which can be readily smelted in a conventional smelter. In addition, Rath is not in any way concerned with solving the technical problems associated with recovering metal values from an extremely inert composition which is designed to resist the harsh conditions utilized in aluminum smelting.
Kapanen et al. in U.S. Pat. No. 4,029,494 disclose a process and apparatus for recovering noble metal values from anode slime produced in an electrolytic copper process. The anode slime containing the recoverable noble metals is subjected to a smelting procedure. Kapanen et al. do not disclose or suggest using their procedure to recover metal values from anodes which comprise cermet. In addition, Kapanen et al. are not in any way with solving the technical problems noted above with respect to recovery of metal values from inert cermet material which is designed to withstand the harsh conditions in aluminum smelting.
Sancinelli in U.S. Pat. No. 5,186,740 discloses the pretreatment of scrap prior to a smelting procedure in which metal values are recovered from the scrap. The pretreatment includes reducing the size of the scrap before it is introduced into a smelter and separating components such as organic materials from the scrap prior to the smelting procedure. Sancinelli does not disclose or suggest any process for recovering metal values from inert anodes which comprise cermet. Furthermore, since Sancinelli is not concerned with the recovery of metal values from cermet, he does not address any of the unique problems associated with recovery of metal values from inert cermet which is specifically designed to withstand the harsh conditions in aluminum smelting.
Elmore et al. in U.S. Pat. No. 4,118,219 disclose a process in which components of lead-acid batteries are subjected to a smelting procedure for the recovery of metal values therefrom. In this procedure a solid metal fraction is isolated and sent to a refinery where it is dried, melted and/or smelted and refined to produce lead alloys which can be re-used in new batteries. Elmore et al. disclose the use of flux in the smelting procedure and further disclose the use of a carbon additive as a reductant in the smelting procedure. However, Elmore et al. do not disclose or suggest the recovery of metal values from inert anodes which comprise cermet and they are not in any way concerned with overcoming the above-noted technical problems associated with recovery of metal values from such an inert material like cermet.
Ogawa et al. in U.S. Pat. No. 4,274,785 disclose the introduction of anode scrap into a converter furnace. The anode scrap functions as a cooling material when it is introduced into the furnace. Ogawa et al. do not disclose or suggest the recovery of metal values from inert anodes which comprise cermet and they do not address any of the above-noted technical problems associated with recovery of metal values from such an inert material.
U.S. Pat. Nos. 3,393,876 and 3,689,253 are of additional interest since they disclose a smelting procedure for the recovery of lead from batteries.
None of the above-noted references address the unique problems associated with the recovery of metal values from cermet material which is designed to withstand the harsh conditions within an aluminum smelter and none of these references disclose or suggest the formation of a cermet material in a form from which metal values can be recovered under metal recovery conditions in a smelter.
It is possible to separate elemental metal from other components, but such separation techniques are not suitable for the recovery of the metal values from cermet and furthermore these techniques do not recover metal values from metal compounds found in the cermet.